1. Field of the Invention
The present invention relates to a method of preparing a polycarbonate resin, and more particularly, to a method of enhancing the thermal stability of polycarbonate by removing the reactivity of a hydroxyl group, which is a polycarbonate terminal group, by reacting the terminal group with an end terminator.
2. Description of the Related Art
A polycarbonate is obtained through the condensation reaction between an aromatic dihydroxy compound, which is a main source material, and phosgene or an aromatic carbonate compound. However, when the reactivity of its terminal group is not removed during preparation of a polycarbonate resin or at the last stage of the preparation, physical-property degradation and discoloration occur due to a molecular weight reduction when the polycarbonate resin is processed at a high temperature.
There is a conventional method of adding low-molecular weight alkylphenol during preparation of polycarbonate resin to improve the thermal stability of the resin. U.S. Pat. No. 4,269,964 discloses a method of reacting the reactive group of growth chain terminals of polycarbonate with alkylphenol acting as an end terminator during interfacial polymerization involved in the preparation of polycarbonate to improve thermal stability against, for example, hydrolysis occurring when polycarbonate contacts high-temperature moisture. However, when this method is used in a non-phosgene process in high-temperature, low-pressure reaction conditions, since the boiling point of alkylphenol used as an end terminator is very low at the reaction conditions, a large amount of alkylphenol is removed through vaporization. Therefore, a larger amount of the end terminator than required to remove the reactivity of the terminal group has to be used, and the vaporized unreacted alkylphenol has to be recovered.
In U.S. Pat. No. 6,620,902, the reactivity of polycarbonate terminal groups is removed by adding a low-molecular weight aromatic hydroxy compound during a post-high temperature process. This reaction is performed under a pressure to prevent vaporization of the low-molecular weight aromatic hydroxy compound. However, although the vaporization of the low-molecular weight aromatic hydroxy compound can be reduced to some extent when the reaction is performed under a pressure, it is not easy to remove the reaction byproducts, thereby making it difficult to remove the reactivity of the terminal groups.
U.S. Pat. No. 6,500,914 discloses a method of removing the reactivity of polycarbonate terminal groups using a high-boiling point aromatic carbonate compound as an end terminator. It is very easy to remove the reactivity of the polycarbonate terminal groups because the used end terminator does not vaporize at high-temperature, reduced pressure conditions. However, a process of synthesizing the used end terminator is very dangerous because phosgene is used as a raw material, and thus the production cost increase. In addition, the end terminator costs high.
Therefore, there is a need for a method of effectively removing the reactivity of polycarbonate resin terminal groups even under high-temperature and reduced pressure conditions using a relatively cheaper, easy-to-prepare end terminator to improve the thermal stability of the polycarbonate resin.